The present invention relates to the curing of coatings and inks, and more particularly, to a light delivery system for curing of photo initiated coatings and inks.
Clear or pigmented coatings and inks are typically used in the printing and painting industries. In one particular application in the printing industry, a web of cellulosic and synthetic material such as paper or the like is printed with ink. Commonly, the ink includes a photo initiator which is responsive to a particular wavelength of light. The photo initiator initiates the curing process of the ink when the ink is exposed to an appropriate wavelength of light. Such a process is commonly known as photo polymerization. Typically, the wavelength associated with the photo initiator is in the ultraviolet range. Photo initiators have become common in the printing industry and in many applications have replaced water and solvent based inks, varnishes and coatings all of which give off volatile organic compounds during the drying process.
It is known in the printing industry to use a medium pressure mercury arc lamp or the like for the curing of the ink or other polymer containing a photo initiator. These lamps generate surface temperatures as much as 750.degree. C. or greater. Additionally, only about 25% of the light emitted from these lamps is usable light in the photo polymerization process. The remaining light is emitted in the form of heat, white light or the like. The tremendous heat generated by the mercury arc lamps has proven detrimental to the printing process because it may damage the printing web and require a cooling system such as a forced air cooling system. Chill drums and methods of cold curing have been used to combat the undesirable effects of the heat generated by the mercury arc lamps. Unfortunately, these methods are expensive.
An additional disadvantage of mercury arc lamps in these applications is that the lamps emit light at wavelengths at less than 300 nanometers (nm) which in turn generate ozone that is disbursed to the atmosphere through the forced air cooling system or other ventilation apparatus.
One alternative to the mercury arc lamps that has been proposed is the use of wave length specific light to initiate the photo polymerization process in the printing industry or other applications. The advantages of wave length specific light for the photo polymerization process include the ability to precisely deliver the wavelength of light that activates a particular photo initiator without requiring the excess heat or energy generated by typical mercury arc lamps. As a result, excess or unused light is not generated and the excessive heat associated with the use of mercury arc lamps is thereby avoided. As a result, chill drums or other cooling devices are typically not required with the use of wave length specific light. Additionally, wave length specific light sources such as lasers or the like do not lead to the generation of environmental contaminates such as volatile organic compounds and ozone.
However, limitations on the use of wave length specific laser light to initiate the photo polymerization of inks on a moving web include the inability to accurately deliver the laser light with sufficient power to the moving web. To effectively activate the photo initiator in the inks requires a minimum amount of energy to be delivered to the ink at the specific wavelength. Past systems which use a wave length specific laser light source for curing photo initiator inks project the light directly from the laser or other light source onto the moving web in a pulsed or continuous (CW) operation. One such application is disclosed in European patent application Ser. No. 202,803 published Nov. 26, 1986, the disclosure of which is hereby incorporated by reference. Another known method for directing the laser light onto the moving web is to scan the laser light beam across the width of the moving web.
Commonly, the print web containing the inks and dyes moves at speeds as great as 2,000 feet per minute or more and the web is typically at least 1.0" in width. To achieve the required dwell time of the laser light on the inks to produce a sufficient amount of energy to initiate the photo polymerization process severely limits the speed of the moving web. Additionally, the combinations of scanning the laser light source across the width of the web in order to expose the entire web to the light source and/or pulsing the laser light source onto the web further limits the speed of the web.
As a result, even though laser light for curing photo initiator inks minimizes the heat generated in the process and any resulting damage to the web as a result of the heat while eliminating the need for chill drums or other cooling devices, the production capability of the printing system is limited as a result of restrictions on the speed of the moving web to achieve adequate dwell time and energy delivered to the ink.